Separation processes using molecular sieves



' July 12, 1966 A. A. YEO ET AL SEPARATION PROCESSES USING MOLECULAR SIEVES Filed June 15; 1963 A/OUPS 0N Shem/w INVENTORS ALAN ARTHUR YEO ROGER TEMPLETON LEWIS MOWLL United States Patent 3,260,667 SEPARATION PROCESSES USING MOLECULAR SIEVES Alan Arthur Yeo and Roger Templeton Lewis Mowll,

Sunbury-on-Thames, Middlesex, England, assignors to The British Petroleum Company Limited, London,

England, a British joint-stock corporation Filed June 13, 1963, Ser. No. 287,718 Claims priority, application Great Britain, June 14, 1962, 22,894/62 2 Claims. (Cl. 208310) This invention relates to separation processes using molecular sieves and particularly to processes for the separation of straight-chain hydrocarbons from petroleum fractions.

It is well known that certain natural and synthetic zeolites have the property of preferentially absorbing certain types of hydrocarbons. These Zeolites, known as molecular sieves, have crystalline structures containing a large number of pores of uniform size. In different zeolites, these pores may vary from 4 A. to 15 A. or more in diameter, but in any one zeolite the pores will be of substantially uniform size.

It has previously been proposed to treat petroleum fractions ranging from gasoline to gas-oils and higher with molecular sieves having pore diameters ranging from 4 A. to 15 A. In order to separate straight-chain hydrocarbons from branched-chain and/ or cyclic hydrocarbons a molecular sieve having pore diameters of 5 A. is suitable. Such a process may be used to recover a denormalized fraction, for example gasoline of higher octane number due to the removal of low octane normal paraflins. The absorbed straight-chain material may also be recovered as a product if desired. For example, a process suitable for separating normal paraffins from petroleum fractions boiling within the C -C range is described in our copending application No. 3,965/ 61.

According to the present invention a process for separating straight-chain hydrocarbons from petroleum fractions boiling above C and containing molecular sieve deactivating hydrocarbons comprises fractionating the feedstock to obtain a lower-boiling fraction substantially free of deactivating hydrocarbons and at least one higherboiling fraction containing deactivating hydrocarbons and treating each fraction successively, inascending order of boiling range, with a 5 A. molecular sieve to extract straight-chain hydrocarbons.

It has been observed that, under equilibrium conditions, the affinity of n-parafiins for molecular sieves increases with increasing molecular weight. Petroleum fractions generally contain small quantities of hydrocarbons which boil outside the nominal boiling range of the fraction, for example up to 0.1% weight of C hydrocarbons may be present in a nominal C -C range gas oil. It is believed that the deactivation rate is at least partially due to the accumulation of high molecular weight hydrocarbons in the sieve pores, these high-boiling hydrocarbons being less readily displaced during the normal desorption stage. In the present application, molecular sieve deactivating hydrocarbons means these high-boiling straightchain hydrocarbons which may, for example, be C hydrocarbons or higher.

3,260,667 Patented July 12, 1966 ice It is desirable to ensure that substantially all the highboiling deactivating hydrocarbons are obtained in the higher boiling fraction but at the same time to ensure that as much as possible of the lower-boiling, non-deactivating material is obtained in the lower-boiling fraction. Preferably, therefore, the feedstock is fractionated at a point within the range 220 to 300 C., and particularly just below the initial boiling point of n-C hydrocarbons, i.e., in the region of 300 C.

By fractionating the feedstock and treating each of the fractions in ascending order of boiling range with a molecular sieve according to the present invention, a higher overall yield of straight-chain hydrocarbons is obtained than would be obtained either by treating the fractions in descending order of boiling range or by treating the unfractioned feedstock. This is due to the fact that during the treatment of the lower-boiling fraction the sieve is not deactivated to any great extent and, in addition, to the fact that the sieve has a higher initial capacity when treating the higher boiling fraction than when treating either heavier boiling fractions or the unfractionated feedstock. The initial sieve capacity and the deactivation rate of the sieve are substantially independent of the amount of lower-boiling hydrocarbons in the feed so that the initial capacity and the deactivation rate are the same when treating a higher-boiling fraction containing all the deactivating hydrocarbons as when treating the unfractionated feedstock.

The method of the present invention may be employed to treat any hydrocarbon feedstocks, which contain deactivating hydrocarbons, for example boiling up to C or higher, and any appropriate molecular sieve treatment may be used. A particularly suitable molecular sieve treatment for hydrocarbon feedstocks boiling in the C -C range is described in our copending U.K. application No. 3,965/ 61. This process is isothermal and comprises contacting the fraction with a 5 A. molecular sieve in a first stage to absorb straight-chain hydrocarbons, contacting the sieve with a purging medium in a second stage to remove material adsorbed on the surface of the sieve or held interstitially between sieve particles, desorbing the absorbed hydrocarbons in a third stage using either n pentane or n-butane as desorbing medium, all three stages being in the vapour phase, the pressure in the desorption stage being equal to, or greater than, the pressure in the absorption stage, and recovering the desorbed straight-chain hydrocarbons in a purity of at least weight. Preferred process conditions include a temperature within the range 300-450 C., preferably 350390 C., and a pressure within the range 0300 p.s.i.g., preferably -150 p.s.i.g.

The invention is illustrated by means of the following comparative example:

A 180-340 C. petroleum distillate was fractionated to give a 180-265 C. fraction containing 0.01% weight of sulphur and a 220-340 C. fraction containing 0.025% weight of sulphur. First the lighter fraction and then the heavier fraction was treated for the recovery of n-paraffins in a bed comprising 2600 grams of 5 A. molecular sieve in a three stage cyclic system under the conditions set out in the following table:

180-265 0. fraction 220-340 C. traction Stage Agent Temper- Pressure, Space Period, Temper- Pressure, Space Period, aEu-e, p.s.i.g. Velocity mins. ague, p.s.i.g. Velocity mins.

Feed 0.6 LHSV 0.6 LHSV. Absorption. {Nitrogenun 350 150 GHSV a 380 s Purge Nitrogen 350 120 GHSV 6 380 125 120 GHSV- 6 Desorpti0n n-Pentane. 350 150 1.0 LHSV 12 380 125 1.0 LHSV 12 The process was repeated under the same conditions only this time the heavier fraction being treated first.

The variation in sieve capacity in each case is indicated in the accompanying graph.

It is seen from the graph that by treating the lowerboiling fraction first, a considerably higher yield of nparaffins Was obtained as follows:

Light finaetion followed by heavy fraction 33,700 grams.

Heavy firaotion fol-lowed by light firaction 31,000 gnams.

Thus an increase in n-paraffin yield of 2700 grams is obtained by treating the lighter fraction first.

We claim:

1. A process for separating with a high recovery yield of straight-chain hydrocarbons from petroleum fractions boiling in the C -C range and containing molecular sieve deactivating hydrocarbons comprising fractionating at a temperature Within the range 220300 C. the feedstock to obtain a lower-boiling fraction substantially free of deactivating hydrocarbons and at least one higherboiling fraction containing deactivating hydrocarbons and treating each fraction successively, in ascending order of boiling range, with a 5 A. molecular sieve to extract straight-chain hydrocarbons.

2. A process as claimed in claim 1 wherein the feedstock is fractionated at a temperature of the order of 300 C.

References Cited by the Examiner UNITED STATES PATENTS 3,160,581 12/1964 Mattox et al 208310 ALPHONSO D. SULLIVAN, Primary Examiner. 

1. A PROCESS FOR SEPARATING WITH A HIGH RECOVERY YIELD OF STRAIGHT-CHAIN HYDROCARBONS FROM PETROLEUM FRACTIONS BOILING IN THE C10-C20 RANGE AND CONTAINING MOLECULAR SIEVE DEACTIVATING HYDROCARBONS COMPRISING FRACTIONATING AT A TEMPERATURE WITHIN THE RANGE 220-300*C. THE FEEDSTOCK TO OBTAIN A LOWER-BOILING FRACTION SUBSTANTIALLY FREE OF DEACTIVATING HYDROCARBONS AND AT LEAST ONE HIGHERBOILING FRACTION CONTAINING DEACTIVATING HYDROCARBONS AND TREATING EACH FRACTION SUCCESSIVELY, IN ASCENDING ORDER OF BOILING RANGE, WITH 5 A. MOLECULAR SIEVE TO EXTRACT STRAIGHT-CHAIN HYDROCARBONS. 